It's freezing and maybe it rained and then the freeze came. These are the perfect conditions for a frozen lock.

If you have an extension cord and a hair dryer you can easily remedy the problem. Warm the lock and then insert key. Use WD-40 to remove any moisture left in the lock.

Lock De-Icer is available at all hardware or locksmith stores. You must spray it all the way back in the keyway. Then it usually takes a few minutes to work. The alcohol will melt the ice, but it also removes the lubricant. Make sure to lubricate well whenever you use a de-icer spray.

Sometimes heating the key and inserting it in the keyway will work. It will probably take several tries to get the inside of the lock to warm up.

Finally, you can call a locksmith who will have a good de-icer and lubricant on hand. However, if it is a good freeze there may be a waiting line.